The present invention relates to semiconductor devices in general and more particularly to a semiconductor device using a barrier layer between the gate electrode and the semiconductor substrate of the device.
Wireless devices such as cellular phones are continually requiring higher efficiency power amplifier improvements. Current cellular phones generally use a power field effect transistor (FET) for controlling power consumption. In a radio frequency (RF) power amplifier, such as a metal semiconductor FET (MESFET) used in a cellular phone, the maximum power that the power transistor in the amplifier is capable of supplying is determined by the highest input power that causes the transistor to become forward-biased. The gate voltage at which the transistor becomes forward biased is primarily determined by the characteristics of the gate metal and semiconductor interface, which is directly related to the Schottky barrier height of the interface.
Current power transistors, for example as used in high performance cellular phones, are typically heterostructure devices using gallium arsenide. However, such devices typically become forward biased at fairly low gate voltages, for example about 0.5 volts. It would be preferable if such devices became forward biased only at voltages substantially greater than 0.5 volts. In order to achieve such greater voltages, the barrier height of the device would need to be increased.
In addition to the above, the MESFETs and high electron mobility transistors (HEMTs) typically used in power amplifiers in high performance cellular phones are depletion-mode devices, which have a negative threshold voltage, and require that the cellular phone include the extra components of a negative voltage generator, extra capacitors to work with the generator, and a p-type metal oxide semiconductor (PMOS) transistor drain switch to more completely shut off power consumption that occurs due to leakage of the power transistor. Such leakage may result in incomplete turn-off of the power amplifier and increased battery or other power supply usage. An advantage, however, of such depletion-mode devices is that they exhibit high saturation drain current, low on-resistance, and high gain.
In contrast to depletion-mode devices, enhancement-mode FETs do not require the extra components above. However, the performance of such enhancement-mode devices is not as desirable as that of the depletion-mode devices. It would be desirable to have a power transistor that combines the performance benefits of depletion-mode devices with the positive threshold voltage of enhancement devices. This would eliminate the extra physical area and expense associated with the extra components described above.
Accordingly, there is a need for a semiconductor device for use in RF power amplifier applications that becomes forward biased at substantially higher gate voltages than current depletion-mode devices and that operates in an enhancement mode.